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Peterbilt Truck No Longer Cools

Big Pete on Wed August 08, 2012 11:12 PM User is offline

Year: 2006
Make: Peterbilt
Model: 387
Engine Size: 15L
Refrigerant Type: R134a
Ambient Temp: 85
Pressure Low: 91
Pressure High: 100
Country of Origin: United States

Hello -

The A/C was working fine when all of a sudden it stopped working.

When I got home, my buddy put on his gauges and it was 100# on the high side and 91# on the low side.

I know when he recharged it earlier this year it was 165# high and 40# low.

If it is now 100/91 do we just have a leak now?

It would not let us add any new freon. What is the significance of that?

AutoCool on Thu August 09, 2012 5:50 AM User is offline

Big Pete, what compressor model is on your truck? Can you read the label?

Big Pete on Thu August 09, 2012 8:31 AM User is offline

It's a Sanden U 4731

Edited: Thu August 09, 2012 at 9:29 AM by Big Pete

bohica2xo on Thu August 09, 2012 10:57 AM User is offline

Is the clutch actually engaging? The flat plate on the front of the drive pulley should be turning when the clutch is engaged.

.

-------------------------
"Among the many misdeeds of the British rule in India, history will look upon the act of depriving a whole nation of arms, as the blackest."
~ Mahatma Gandhi, Gandhi, An Autobiography, M. K. Gandhi, page 446.

Big Pete on Thu August 09, 2012 12:41 PM User is offline

Yes the clutch is engaging. But again, the pressure on the high and low side are almost the same. 100/91

Upon further observation, I see that the expansion valve is leaking green dye out the side of it My buddy said this is probably leak detector dye.

Would a bad expansion valve cause the pressures to be 100/91 when running?

Edited: Thu August 09, 2012 at 12:45 PM by Big Pete

bohica2xo on Thu August 09, 2012 1:05 PM User is offline

I would expect a lot more differential pressure across the TXV, even if it is stuck wide open.

Probably a dead compressor. Diesels are had on accessories, and class 8 trucks pick up a lot more miles than a passenger car in the same period of time.

I would check the whole system for leaks with a UV light. Clean the TXV with some brake cleaner, and re-check it a day later. If it is leaking you will see fresh dye.

B.

-------------------------
"Among the many misdeeds of the British rule in India, history will look upon the act of depriving a whole nation of arms, as the blackest."
~ Mahatma Gandhi, Gandhi, An Autobiography, M. K. Gandhi, page 446.

AutoCool on Thu August 09, 2012 1:20 PM User is offline

That compressor is a SD7H15. Fixed displacement. If the expansion valve is stuck open then pressures would be just about equal. Is your buddys gauge set calibrated? Maybe the pressures actually are equal?

I would recommend recovering the refrigerant and pull the suction and discharge lines off the compressor. If you block the ports with your finger and spin the compressor hub it should draw a vacuum/build pressure almost right away. It may have stripped gears, there is actually a picture of one with stripped gears in the Sanden compressor manual.

Might be easier to inspect the expansion valve first, I've never really seen a Peterbilt AC system. Good luck.

NickD on Thu August 09, 2012 3:36 PM User is offline

91/100 with a spinning compressor is a problem. We can speculate on the cause, a broken crankshaft, reed valve, expansion valve, etc. Just tells you the system has to be open and the individual components tested. Practically all throwaway stuff, get some decent replacements.

mk378 on Thu August 09, 2012 6:29 PM User is offline

I agree the compressor is likely shot. Expansion valves are built to only open to a certain extent, which matches what a good compressor at cruise rpm can do. With the compressor disconnected, put your thumb over the discharge fitting and spin by hand to see if it builds any pressure at all.

Of course you need to fix the leak too.

ice-n-tropics on Thu August 09, 2012 8:17 PM User is offline

Big Pete
Some potential causes of Ps=91 w/armature engaging pulley rotor (w/o a auxilary sleeper evaporator):
1) Service port not open to suction gage
2) Reed valves broken.( Stripped gears are extremely rare and only after total compressor grenade and w/ loud grinding noise)
3)Block type TXV stuck wide open due to internal failure, but discharge pressure would be higher due to high refrigerant mass flow

Low charge due to leak at TXV would result in abnormally low suction pressure. Maybe the Ps is wrong as in (1) above
If you block the total condenser frontal area w/ cardboard and the discharge pressure rises to 250 psi at 95F ambient, the SD7 is probably OK

hotrodac

-------------------------
Isentropic Efficiency=Ratio of Theoretical Compression Energy/Actual Energy.
AMAZON.com: How To Air Condition Your Hot Rod

bohica2xo on Fri August 10, 2012 2:23 AM User is offline

I find it fascinating that people think there is a huge opening in a TXV.

When that valve has heat soaked underhood, it is open as far as it will go. Yet when you jump in your car and turn on the A/C, the high side jumps right up to 200 or 300 psi depending on ambient temps, and the low side drops below static pressure - in seconds. If they system could not build that differential pressure, it would never cool.

When you have a 10 psi differential across the system with a full charge, the pump is not pumping.

.

-------------------------
"Among the many misdeeds of the British rule in India, history will look upon the act of depriving a whole nation of arms, as the blackest."
~ Mahatma Gandhi, Gandhi, An Autobiography, M. K. Gandhi, page 446.

NickD on Fri August 10, 2012 6:12 AM User is offline

Mitch reported, miss that guy, jury rigging a high pressure gauge to the output, could hit 120 psi by hand turning it. Never went that scientific, either on the input with a thumb to check for a vacuum or on the output to check for pressure.

Major problems with expansion valves is getting plugged up or a broken capillary tube where it runs wide open. When the AC is first turned on, always wide open, if that equalized the high and low side pressure, system would never cool down. Even if brand new. Typically, is a cycling system, under cooler ambient conditions, closes to reduce refrigerant flow to reduce the degree of cycling.

We could also discuss clutch gap as an issue, but reported that is working fine. Reed valves also tend to get gummed up preventing full closure and can work one second and quit the next.

ice-n-tropics on Fri August 10, 2012 3:41 PM User is offline

Yep,
Mobil TXVs vary on wide open orifice to about .107" max, .093" typical and sometimes only .078"

-------------------------
Isentropic Efficiency=Ratio of Theoretical Compression Energy/Actual Energy.
AMAZON.com: How To Air Condition Your Hot Rod

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